1. Field of the Invention
The present invention relates to liquid crystals, and more particularly, to a method for recovering the electrical properties of a degraded liquid crystal.
2. Description of the Prior Art
Because of the problems such as the reduction of voltage holding ratio, increase in threshold voltage, image sticking, gray-level shift, image flickering, and the slowdown of response, the field-screening effect originating from impurity ions within a liquid-crystal (LC) cell has long been a critical issue in the liquid-crystal display (LCD) technology.
For example, Perlmutter et al. studied the performance degradation of LC cells by analyzing the time evolution of optical and integrated current vs. voltage hysteresis curves when the test cells were subjected to a direct current (DC) bias. They found that the performance of the LC cells is degraded due to permanent increases in mobile ion populations in the LC bulks, and suggested that this permanent increase is due to selective adsorption by the alignment layers of ions of a single charge sign combined with the presence of a neutral ionizable species in the liquid crystal. [S. H. Perlmutter, D. Doroski, and G. Modder, “Degradation of liquid crystal device performance due to selective adsorption of ions,” Applied Physics Letters 69(9), 1182-1184 (1996).] For example, Palomares et al. investigated the influence of ions dissolved in liquid crystals by checking the optical response of a nematic liquid crystal submitted to a periodic square wave. They showed that various densities of ions and various properties of the dielectric layer deposited on the electrodes result in different regimes having different optical phase. In addition, they investigate the role of the mobility of the ions in the phenomenon and supposed that the diffusion current is negligible with respect to the drift current. [L. O. Palomares, J. A. Reyes, and G. Barbero, “Influence of ions on the dynamical response of a nematic cell submitted to a periodic external field: Role of the ionic impurities,” Physics Letters A 333, 157-163 (2004).] Tsevetkov et al. observed a carrier charge transfer and its influence on electro-optical response for nematic liquid crystal with positive dielectric anisotropy (Δε>0) in which ions with known parameters (Cu+) were injected. The experimental results showed that the presence of the ions may increase the electro-optical response due to hydrodynamic component, and the developed technique could be applied to measure the mobility of the ions. [V. A. Tsevetkov and O. V. Tevetkov, “Ions influence on electrooptical characteristics of NLC,” Molecular Crystals and Liquid Crystals 368, 625-632 (2001).] Colpaert et al. proposed an appropriate measuring technique to determine the ion source and characterize the ions in the liquid crystal. They suggested that the ion contamination in twisted nematic liquid crystals should be limited to assure good electro-optical performance of AM-LCDs. [C. Colpaert, B. Maximus, and A. de Meyere, “Adequate measuring techniques for ions in liquid crystal layers,” Liquid Crystals 21(1), 133-142 (1996).]
To solve the problems caused by the ions, current strategy is to employ high-resistivity LC materials with a low concentration of impurity ions and to address the displays in alternating current (AC) to avoid surface polarization, which would otherwise lead to the field screening and, in turn, degrade the electro-optical performance of the devices. Recently, observations on voltage-dependent transmittance, voltage-dependent capacitance, dynamical optical response, and behavior of transient current in a polarity-reversed field have invariably shown that the inclusion of an adequate amount of carbon nanotubes dramatically reduces the abominable mobile ions in cells consisting of a low-resistivity LC mixture [W. Lee, C.-Y. Wang, and Y.-C. Shih, “Effects of carbon nanosolids on the electro-optical properties of a twisted nematic liquid-crystal host,” Applied Physics Letters 85(4), 513-515 (2004); H.-Y. Chen and W. Lee, “Electro-optical characteristics of a twisted nematic liquid-crystal cell doped with carbon nanotubes in a dc electric field,” Optical Review 12(3), 223-225 (2005); W. Lee, J.-S. Gau, and H.-Y. Chen, “Electro-optical properties of planar nematic cells impregnated with carbon nanosolids,” Applied Physics B: Lasers and Optics 81(2/3), 171-175 (2005); W. Lee and Y.-C. Shih, “Effects of carbon nanotube doping on the performance of a TN-LCD,” Journal of the Society for Information Display 13(9), 743-747 (2005); H.-Y. Chen and W. Lee, “Suppression of field screening in nematic liquid crystals by carbon nanotubes,” Applied Physics Letters 88(22), 222105-1-3 (2006); H.-Y. Chen, W. Lee, and N. A. Clark, “Faster electro-optical response characteristics of a carbon-nanotube-nematic suspension,” Applied Physics Letters 90(3), 033510-1-3 (2007); W. Lee and H.-Y. Chen, “Observation of transient current in a nanotube-doped liquid-crystal cell induced by a polarity-reversed field,” Japanese Journal of Applied Physics 46(5A), 2962-2967 (2007); K.-X. Yang and W. Lee, “Temperature-dependent electric characteristics in an E7/CNT colloid,” Molecular Crystals and Liquid Crystals 475(1), 201-208 (2007); W. Lee, H.-Y. Chen, and Y.-C. Shih, “Reduced dc offset and faster dynamic response in a carbon-nanotube-impregnated liquid-crystal display,” Journal of the Society for Information Display 16(7), 733-741 (2008); M. Rahman and W. Lee, “Scientific duo of carbon nanotubes and nematic liquid crystals,” Journal of Physics D: Applied Physics 42(6), 063001-1-12 (2009).]
In the relevant art, manufacturers and researchers have made many efforts to eliminate the influence of ions in liquid crystals. For example, one solution has been to request that the manufacturers produce liquid crystals of higher purity and higher reliability; other solutions have been to develop new LC materials or new synthesis methods, or to select more appropriate liquid crystals for given devices, such as thin film transistor liquid crystal displays (TFT-LCDs). Unfortunately, although the process capability has been strictly requested and new liquid crystals and new synthesis methods have been developed, unwanted ions are still somehow dissolved in the liquid crystals and, in turn, affect the performance of the resulting display devices.
On the other hand, it is well-known that liquid crystals stored in bottles or vials gradually lose their electro-optical integrity with time, trapping more and more ion impurity in the materials. Although doping carbon nanotubes or nanoclay can dramatically reduce the mobile ions in LC cells, the properties of the liquid crystal are altered by the dopants and thus may hinder the electrical design of the display devices. Moreover, the colloidal stability of the LC cells doped with nanotubes may be damaged after a long-term operation. Besides, altering of the driving waveform of the liquid crystals gradually (e.g. over time) cannot satisfy the more rigorous specifications, including more rigorous image sticking, flicker, and the like. In another aspect, recent liquid crystals with higher purity and superior properties have been made by newer synthesis methods. Although the liquid crystals are superior in purity, lightfastness, thermostability, reliability, and resistivity, the storing environment or the manufacturing environment inevitably contaminates the liquid crystals during the processes, thus degrading the properties of the liquid crystals.
In sum, known prior-art approaches have failed to identify the source of the abominable mobile ions and/or to provide an efficient way to treat the decayed liquid crystals. The conventional and only practical treatment for deteriorated liquid crystals is to discard them, resulting in large unwanted cost to manufacturers for the liquid crystals and display devices. Accordingly, there is still a need to identify the primary cause of unwanted mobile ions in, and thus to provide a more cost-effective treatment of, and even better to recover the properties of, such decayed liquid crystals.